Accelerator radiation source



Sept. 29, 1964 w. E. MOT-r 3,151,243

ACCELERATOR RADIATION SOURCE Filed April l1, 1960 INVENTOR. //ll/M E. M077' United States Patent O Bands Filed Apr. 11, 1960, Ser. No. 21,377 6 Claims. (Si. Z50-84.5)

This invention relates to new and useful improvements in borehole logging apparatus and especially to radiation sources thereof of the type wherein ions are accelerated to strike a target containing nuclei reactive with the accelerated ions to produce the desired radiations. More particularly, this invention relates to such `an apparatus wherein a substantial improvement towards achieving a constant radiation output is obtained by maintaining a pressure differential between the ion accelerating portion of the apparatus and the ion-forming portion of the apparatus in favor of the latter through a novel arrangement of ion source, ion accelerator, and pumping means to be described.

The present application is a continuation-impart of my copending application Ser. No. 580,834, iiled April 26, 1956, issued `on August 1, 1961, as Patent No. 2,994,775, and assigned to the same assignee as the present application. The instant invention is related to the same general class of subject matter as that disclosed in my two similarly assigned applications Ser. No. 580,833, filed April 26, 1956, entitled Borehole Logging, issued August 1, 1961, as Patent No. 2,994,774, and Ser. No. 580,906, filed April 26, 1956, entitled Stabilized Borehole Logging, issued August 1, 1961, as Patent No. 2,994,776.

Broadly the invention relates to a neutron source adapted to be used in a borehole and comprising a linear ion accelerator, a source of ions, means for introducing material to be ionized into the ion source, `and a pump connected to the ion accelerator in such manner as to maintain a pressure differential between the ion accelerator and the ion source in favor of the latter. The abovementioned copending applications disclose automatic controls on the rate at which material to be ionized is admitted to the ion source, and to automatic control of the ion source. The present application relates to a physical arrangement of the elements of a linear ion accelerator whereby stability of pressure in the respective elements is maintained thereby substantially increasing stability of output radiation with or without the use of such additional controls as disclosed in the above-mentioned applications.

The invention also entails an arrangement of -a sour of penetrating radiation of the character above specified together with a radiation detector such that they can both be contained within a housing adapted to be lowered Within a borehole, and also such that the radiation detector and the target component of the source of penetrating radiation can, if desired, be placed in proximity to each other that is limited substantially solely by the amount of shielding material desired to be interposed therebetween.

A primary object of the subject invention is the provision of a linear ion accelerator capable of insertion into a borehole and producing a neutron ux having stability heretofore unattainable in borehole logging apparatus.

The invention will be fully comprehended in the light of the following description of a preferred embodiment of the invention taken together with the accompanying drawings illustrative thereof, wherein:

FIGURE 1 is a diagrammatic illustration of the manner of use of the preferred embodiment of the invention;

FIGURE 2 is a diagrammatic illustration of the disposition of the respective elements of the source of penetrating radiation and the radiation detector;

FIGURE 3 is a schematic representation of the electrical system associated with the elements of the source of penetratingradiation; and

FIGURES 4, 5, 6 and 7 are cross-sectional views of FIGURE 2 taken at the locations indicated therein.

Referring to the drawings -and to FIGURE 1 in parV ticular, the numeral 10 designates a borehole Which can be cased or uncased. The numeral 12 designates a housing supported by a cable 14 of the conventional type that includes provision for electrical conductors indicated generally by 16 in FIGURE 2, for vertical movement within the borehole. The supporting cable 14V passes over a supporting pulley 18 and is wound on a reel 20 above the earths surface 22. Means, not shown, are provided for driving and braking the reel 20 for raising and lowering the housing 12 i-n a conventional manner.

The interior of the housing is shown 4in schematic detail in FGURE 2. Preferably resiliently supported within the housing 12 is a vacuum-tight container 24 that is preferably metallic, and which like the housing 12 is constituted of a material that is substantially transparent to the penetrating radiation that is produced by elements within the casing 24 which are to be presently described. inasmuch as the expression penetrating radiation as used in this specification and in the accompanying claims is meant to include neutrons as well as gamma rays or the combination of the two, the casing 24 can be conveniently fabricated of aluminum, and the housing 12 of steel. Such materials are suiiiciently transparent to the penetrating radiations and afford sutlicient structural strengths for their respective purposes.

Disposed in spaced relation within the casing 24, and supported therein by means 23 and 25 is a hollow generally cylindrical container 26 formed of electrical insulative material such as glass that is closed at its opposite ends; The container 26 is provided with side openings 72 and 74 whose purpose Will be explained later. Theoutside diameter of inner container 26, as compared with the inside diameter rof outer container 24, is such that the annular space 27 between them is large. Preferably the annular space Z7 has a transverse area rat least as large as the transverse area of the inside of the container 26, as is illustrated in FIGURES 4 to 7. The large annular space 27 cooperates wih the openings 72 and 74 in a manner to be explained later.

The container 26 encloses an ion source designated generally at 28; means designated generally at 29 for introducing a material to be ionized into the ion source 28; a linear ion accelerator designated generally at 32 and including a target 34; and a pump 36 preferably of the ionic type for maintaining a high vacuum Within the accelerator 32.

The ion source 2b comprises cathodes 38 and 40, and a hollow cylindrical anode 42. The cathode 38 is formed with a plurality of openings 44 therethrough so that materialto be ionized can pass into the ion source 28. The cathode 38 is insulatingly supported upon the inner surface of the container 26 as shown. The cathode 40 is insulatingly supported upon the inner surface of the container 26 by a vacuum-tight sealing material 46. The cathode 4t) is provided with a small central opening 48 that constitutes a restricted exit by means of which ions may pass from the ion source 28 into the accelerator 32. Because of the seal 46, the only exit from the ion source 28 into the accelerator 32 is through the exit 48. The anode 42' has -its opposite ends spaced from the cathodes 38 and 40 and is insulatingly supported upon the inner surface of the container 26. The insulating supports for elements 3S, 4d, and 42 are preferably made of a material having a high dielectric strength, such as glass, ceramic, or the like. The cathodes 38 and 40 are preferably formed of or include magnetized material so that a strong magnetic eld passes from one of the cathodes 38 and 40 to the other through the anode 42 so as to prolong the flight path of electrons during operation of the ion source 28 and thereby increase the occurrence of ionizing events. It is understood that appropriate electrical connections (not shown in FIGURE 2) are brought out through seals in the wall of containers 24 and 26, for the purpose of connecting the elements 3S, 4i?, and 42 to a source of high voltage.

The means 29 for introducing a material to be ionized in/to the ion source 28 includes a Vacuum-tight partition i52, which can be formed of glass or metal, adjacent the lower end of the container 26 so as to dene a reservoir 54 for material to be ionized. The means 29 for introducing material into the ion source 2S includes electrically controllable valve means 30 provided for regulating the rate at which a gaseous material contained within the reservoir 54 is allowed to pass into the ion source 28. Conventional means, not shown, are provided for charging the reservoir 54. Although any type of valve means 30 subject to electrical control for regulating minute gaseous flow rates can be employed, as for example a small spring-pressed needle valve controllably opened by au electric solenoid, the preferred form of valve means is the combination of a palladium thimble 56 that is sealed vacuum-tight in an opening in the partition 52, and an electrical heating element 58 for heating the thimble 56. The reservoir 54 will customarily contain gaseous hydrogen or one of its isotopes, deuterium or tritium, and as is well known, palladium, tantalum, and certain other substances possess to a considerable degree the property of permitting the diffusion of hydrogen or its isotopes therethrough, with such property becoming increasingly more pronounced with increasing temperatures, Also, such preferred form of valve means 30 functions to filter out highmolecular weight impurities. In the claims the expression diffusion barrier is meant to denne a thin section of such substances. Thus hydrogen or one of its isotopes can be introduced into the ion source 28 at a rate dependent upon the temperature of the palladium thimble 56 as controlled by the rate at which electrical energy is supplied to the electrical heating element 58.

The linear ion accelerator 32, comprises in its preferred form a probe or focusing electrode 60, accelerating electrodes 62 and 64, and a target 34. By a linear ion accelerator is meant a linear arrangement of electrodes to which electric potentials are applied for the purpose of accelerating charged particles in a straight line and causing the particles to bombard a target. The probe electrode 60 is a hollow cylindrical sleeve that is necked down and provided with a small central opening 66 located in line with exit 48 in the cathode 40 of the ion source 28. The probe electrode 6@ is insulatingly supported upon the inner surface of container 26 by a vacuum-tight sealing material 50 as indicated in the transverse section shown in FIGURE 4, so that the only entrance into the lower end of accelerator 32 is through the opening 66. The accelerating electrodes 62 and 64 are coaxial hollow sleeves and are insulatingly supported from the inner surface of the container 26 by slender radial supports 63 and 65, preferably three or more in number. Whereas only two accelerating electrodes 62 and 64 are shown in the drawings, it is to be understood that more such accelerating electrodes may be provided when higher accelerating potentials are employed to accelerate particles to correspondingly higher energies. For a reason to be explained later, the respective electrode supports 63 and 65 are made in the form of slender rods or thin axially extending ribs so that they present little obstruction to gas molecules traveling axially in the space 31 between the outside of the electrodes 62 and 64 and the inner wall of the container 26. This is illustrated in FIGURE 5 which is a transverse section through the electrode 62 and its supports as indi- 4 cated in FIGURE 2. The electrode 64 is similarly supported. It is of course understood that appropriate electrical connections (not shown in FIGURE 2) are brought out through seals in the wall of containers 24 and 26 for the purpose of connecting the respective electrodes 60, 62, and 64 to a source of high voltage.

The target 34 is insulatingly supported upon the inner surface of the container 26 by a support 68. The support 68 may comprise a slender rod or bar of glass to the inner end of which the target 34 is fastened, but for purposes of rigidity and strength it is preferred to support the target on three or more such supports in the form of slender rods fastened to the inside wall of container 26 and supporting the target 34 as illustrated in FIGURE 6 which is a transverse section through the target 34 as indicated. An electrical connection (not shown) connected to target 34 is brought out through seals in the containers 4 and 26 so that an appropriate electrical potential can be applied to the target.

The target 34 is arranged to be bombarded by ions accelerated by the accelerator 32, preferably in a diffused manner, and includes atomic nuclei that undergo a nuclear reaction with the bombarding ions to produce the penetrating radiation, which as previously indicated can be either gamma rays or neutrons. Where it is desired that the penetrating radiation be high-energy neutrons, the bombarding ions can be deutrons and the target 34 contains nuclei of tritium, or alternatively, the bombarding ions can be tritons and the target 34 include nuclei of deuterium. With either of such arrangements, where the bombarding ions have been accelerated through a potential difference of about 2O or more kilovolts (higher accelerating potentials being preferred), a nuclear reaction occurs at the target 34 productive of neutrons having about 14 mev.

Where lower-energy neutrons are desired, the bombarding ions can be deutrons accelerated through a potential dilerence of about 20 or more kilovolts (higher accelerating potentials being preferred) and the target 34 contain nuclei of deuterium, in which case neutrons of about 2.5 mev, are produced at the target.

Where the target 34 is to contain either the nuclei of tritium or the nuclei of deuterium, the target can conveniently comprise a plate of tungsten or platinum, coated or plated with a thin layer of zirconium on the side adjacent the accelerator 32 and having tritium or deuterium, as the case may be, adsorbed in the zirconium layer.

Where it is desired that the penetrating radiation be gamma rays, many combinations of types of bombarding ions and types of nuclei to be included in the target will occur to those skilled in the art. For example, the bombarding ions can be protons and the target include nuclei such as 3Li7, 9F19, 6G12, or 6G13. Higher accelerating potentials are required to cause gamma-ray producing reactions with these combinations, than are required for the previously mentioned neutron producing reactions. Accordingly, an appropriate selection of high-Voltage source for the accelerator must be made in view of the fact that for maximum yield protons must be accelerated to energies on the order of about 450 kilovolts to react with 3Li7 to produce 17 meV. gamma rays; 350 kilovolts to react with 9F19 to produce 6 meV. gamma rays; 450 kilovolts to react with 6G12 to produce 2 mev. gamma rays; and 560 kilovolts to react with 6C13 to produce 8- mev. gamma rays. Also, the target can include nuclei of 3Be9 to produce gamma rays of various energies up to about 7 mev. where an accelerating potential of about 1 megavolt or better is available. Lesser accelerating potentials or proton energies can of course be used in such combinations with reduced gamma-ray yield.

A pump 36 is schematically indicated in FIGURE 2 as located substantially on the axis of` container 26 above the target 34. The pump36 is located inside the container 26 in close proximity to the target 34 and is connected to the space around the target in order that the pump will eliect fast evacuation of gas from the accelerator 32 of container 26. Inasmuch as the slender supports of the electrodes 62, 64, and target 34 form only small obstructions to the flow of any un-ionized gas in the accelerator 32, the pump 36 will be effective in maintain.- ing the accelerator evacuated to a pressure sufficiently low so that the mean-free path of gas molecules is large compared to the dimensions of the accelerator 32, whereby the pump 36 will have maximum effectiveness in maintaining a low pressure in the accelerator space between the orilice 66 and the target 34.

The primary purpose of pump 36 is to maintain a very low pressure in the accelerator 32, say on the order of -5 to 10*6 mm. Hg. Though several well-known types of pumps are capable of maintaining such a high vacuum, the stringent space limitations inherent in borehole logging apparatus make it preferable that the pump 36 be of the ionic type. The schematically illustrated ionic pump 36 is sufficiently small in size for borehole operation. lt comprises an electrical heating element for vaporizing substances such as zirconium, titanium, etc., which on condensating on adjacent surfaces, presents a large surface area that strongly adsorbs isotopes of hydrogen. Should the atmosphere within the accelerator 32 contain appreciable amounts of gases unsuited to the use of an ionic pump, as where the bombarding ions have a mass number in excess of three, resort must be made to some other type of high-vacuum pump that possesses the requisite small physical size. As indicated in FIGURE 2, the intake to the pump 36 is in substantially unobstructed pumping communication with the accelerator space around the target 34. The container 26 is provided with side openings at 72 and 74 so that the pump 36 can not only directly evacuate the interior of accelerator 32, but also directly evacuate the space between the cathode 46 and the probe 60. The openings 72 are made so that they have in their aggregate an area at least as large as the area of the inside of container 26 for the purpose of providing maximum pumping communication between the pump 36 and the annular space 27 between the container 26 and the outer sealed container 24. To this end the transverse section through openings 72 is as shown in FIGURE 7. The pump 36 is thus in relatively unobstructed pumping communication with the annular space 27 and can quickly and effectively evacuate the same to substantially the same low pressure as exists in the accelerator 32. The openings 74 are similarly arranged to provide maximum communication between the annular space 76 and the interior region between the cathode 4t) and the probe electrode 60. The openings 74 are similar to the openings 72 illustrated in FlGURE 7, i.e. in their aggregate the openings 72 have an area at least as large as the area of the inside of container 26 so as to provide relatively unobstructed pumping communication between the space intermediate the cathode 40 and the probe 60 and the annular space 27. With this construction anyV uri-ionized gas molecules escaping from the ion source exit 48 Will quickly reach the pump 36 and be evacuated from the accelerator 32, and therefore will not interfere with the free flight of ions moving from the opening 43 to the target 34 during operation of the ion accelerator 32. On the other hand ionized particles emerging from the ion source exit 48 will be accelerated by the electric fields in the accelerator 32 to bombard the target 34.

In order to further increase the eiiiciency of pumping out the accelerator 32, the space between outer container 24 and the lower portion of container 26 (i.e. around the ion source 28 and gas container 29) is sealed off by a vacuum-tight partition 2S which may also serve to support container 26 inside sealed container 24. Similarly the upper portion of container 26 (i.e. around the pump 36) is sealed off by a Vacuum-tight partition 23 which may also serve to support container 26 inside sealed container 24.

The above-described structure is such that the pump 36 will maintain the required low pressure in the ion-v accelerator portion 32 of the apparatus and this structure is particularly advantageous in a borehole device. By placing the lpump 36 on the axis of the apparatus beyond the target 34 the pump does not occupy diametral space which is at a premium in a borehole device, and at the same time the `structure provides for the pump to have substantially unobstructed access to both the target region and the ion-injection region of the accelerator 32 whereby the accelerator is maintained at a low pressure required for efiicient operation. Furthermore by thus maintaining the accelerator 32 substantially evacuated, i.e. at a pressure suiiiciently low that the accelerated ions suffer substantially no collisions during their flight from entrance opening 66 to the target 34, the accelerator will operate at substantially constant output, whereby a substantially constant radiation iiux is obtained from the apparatus.

Attention is now directed to FIGURE 3 wherein the electrical system associated with the previously described electrical elements is schematically illustrated.

A high voltage supply 76 having a suiiicientiy high voltage output for accelerating ions to the energy necessaryv to undergo the nuclear reaction productive of the desired penetrating radiation is provided which can conveniently be of the Van de Graaff or Cockcroft-Walton types, which supply 76 has its negative terminal 77 grounded as at 78 and is connected by a lead Sii to the electrode 64. The electrode 62 and the target 34 are adjustably tapped by-leads 82 and 84 to the positive side of the supply 76, the arrangement being such that the target 34 has a potential sufciently positive with respect to the electrode 614 as to retard bombardment of the latter by electrons emitted by the target 34 during operation.

A further high voltage supply 86 for energizing the ion source is provided, which has its negative terminal' 37 connected to the positive terminal 83 of the supply 76 by a lead 89. The cathode iii and the probe electrode 66 are adjustably tapped to the positive side of the supply 86 by leads 9i) and 92, respectively, with the cathode 46 being connected to the cathode 38 by a lead 94.

The anode 42 is connected by means of leads 96 and 98 through a current-responsive device 169 to the positive terminal 161 of the supply 86. The function of the current-responsive device 166 is to control the rate at which electrical energy is supplied to the electrical heating element 53 for the palladium thimble S6 through leads 102 and N4. The arrangement is such that whenever the current iiowing through the lead 96 falls below a predetermined value, the rate of supply of electrical energy to the heating element 5S is increased, and conversely, wheneverv the current through the lead 96 rises above said predetermined value, the rate of supply of electrical energy to the heating element 58 is decreased. The

- current-responsive device 16) can be a conventional servo system adapted to function as above specified wherein the output of the sensing element thereof is responsive to variationsV of the electrical current iiowing in lead 96 to control by means of an electric motor the supply of electrical energy to the heating element S8 of the thimble 56. For example, though many other forms of servo systems or equivalents thereof will readily occur to those trained in this iield, the current-responsive device can be such as that illustrated in FGURE 2 of US. Patent No. 2,735,943, issued February 21, 1956, to Wright et al., wherein the motor 72 thereof is arranged to drive a variable rheostat controlling the supply of electrical energy to the heating element 58.

The electrical potential established between the anode 42 and the cathodes 38 and rtt is adjusted to be a substantially constant value in the range of say about 200 to about 5000 volts, and the previously mentioned predetermined current in the lead 96 is that amount of current which will ow through the lead 96 when the electrical potential between the anode 42 and the cathodes 3S and 46 has been iixed and the pressure within the ion source has been fixed at a value on the order of 11i-3 to itl-4 mm. Hg.

It will be appreciated by those skilled in the art that at pressures on the order of 104' to l0*4 mm. Hg Within the ion source 2S, and with a substantially xed potential diiierence established between the anode 42 and the cathodes 3S and 40, the value of the ionization current flowing in the ion source 28 as represented by the current flowing in the lead 96 is substantially a linear function of the pressure within the ion source 28. (This, of course, is the principle upon which ionization gauges are based.) Accordingly, the above described function of the current-responsive device 100, the heating element 58, and the palladium thimble 56 is such as to maintain the pressure within the ion source 28 substantially constant. Maintenance of a substantially constant pressure within the ion source 28 assures a substantially constant rate of supply of ions to the accelerator 32. This in turn contributes materially to the attainment of a reasonably constant flux of penetrating radiation output from the target 34.

A source of electrical energy 106 is provided for the ionic pump 36 and is connected thereto by leads 108 and 110, the latter being grounded as shown.

As shown in FiGURE 2, the voltage supplies 76 and 86, the current responsive device 100, and the source 106 are suitably mounted within the housing 12 below the container 26. As will be seen presently this arrangement places the high voltage equipment at a position remote from radiation detection apparatus subsequently to be described. If desired, the source 196 can be mounted in the housing 12 above the casing 24, as the latter operates at or near ground potential.

Disposed within the housing 12 above the container 26 is a radiation detector 114 which can be of any desired type to detect radiations entering the borehole from the earth formations 11S as a consequence, however indirect, of the latter being subjected to penetrating radiation from the target 34. For example, irrespective of the nature of the penetrating radiation produced by the target 34, the radiation detector 114 can be the combination of a scintillation phosphor preferentially sensitive to either gamma-rays or neutrons and a photomultiplier tube; or as a further example, the detector 114 can be a proportioned counter of either the gamma-ray or neutron detecting types. Also the detector 114 can be surrounded with any desired combination of gamma-ray shielding, neutron moderator, neutron absorber, or the like to effect any desired filtering or shielding action so as to obtain detection selectivity and/ or directivity.

Means are also provided for recording the output of the radiation detector 114 or any selected portion of such output with respect to the depth of the housing 12 within the borehole 10, A large number of such means are known which can be used for this purpose, the selection of the specific means to be employed being primarily dependent upon the nature of the information sought. By way of example, the radiation detector 114 is a combination of scintillation phosphor sensitive to gamma-rays and a photomultiplier tube with the output of the detector 114 being fed as indicated at 116 to an amplifier 118. The output of the amplifier 118 is in turn fed to a pulseheight analyzer 122 as indicated at 120. The output of the pulse-height analyzer 122 is fed by means indicated as the electric conduit 16 within the cable 14, and a pickup circuit indicated as 126 connected to the reel 20, to a combination counting-rate meter and recorder shown at 12S, the latter being operatively connected as indicated at 130 to the pulley 18 so as to obtain a record Versus depth. If desired, the pulse-height analyzer 122 can be removed from the housing 12 and included in the surface equipment.

Means, not shown, of conventional character are provided for supplying electric energy through the electric conduit means 16 within the cable 14 to the elements 8 76, S6, 10i), 106, 114, 118, and 122 within the housing 12. if desired, some of such elements can be supplied electrical energization by means of batteries, not shown, disposed within the housing 12.

in order to limit the extent to which penetrating radiation produced by the target 34 can directly reach the radiation detector 114, shielding means 132 can be disposed Within the housing 12 intermediate the target 34 and the radiation detector 114. The character of the shielding means 32 will be evident to those skilled in the art, lead being conventional for use as gamma-ray shielding, and a neutron moderator such as paraiiin backed by cadmium being conventional for use as neutron shielding. Both of such types of shielding can be combined to be effective shielding for a combination of both types of penetrating radiation produced by the target 34.

In view of the preceding detailed description of the apparatus, the overall operation thereof Will be readily apparent. For purposes of describing the overall operation of the apparatus, it will be assumed that the same is being employed to irradiate the earth formations surrounding the borehole 10 with neutrons of about 14 mev. Accordingly, the target 34 includes nuclei of tritium and the reservoir 54 contains deuterium gas. Deuterium ions are fed from the ion source 28 to the accelerator 32 at a substantially constant rate by reason of the previously described control of the pressure in the ion source 28 in response to the current in the lead 96. The deuterium ions are then accelerated by the electrodes 60, 62, and

64 to a sutiicient velocity to undergo a nuclear reaction with the tritium nuclei included in the target 34, such reaction being productive of 14 mev. neutrons. The neutrons thus produced proceed outwardly from the target 34 into the earth formations 115. Radiation of a preselected character, say gamma rays having 6 mev. energy, returning to the borehole 10 from the earth formations 115 as a consequence, however indirect, of the latter being subjected to irradiation by 14 meV. neutrons from the target 34, are detected and recorded versus depth by the elements 114, 118, 122, and 128.

It will be noted that the illustrated and described linear ion accelerator is especially suited to borehole logging apparatus inasmuch as the same will readily admit of the use of high voltages and yet is quite conformable to the small horizontal cross section of space available in any borehole logging apparatus. Also, linear ion acceleration permits the target to be placed in relatively close proximity to the radiation detector where such close spacing is desired.

The advantage of the described mode of maintaining a constant pressure within the ion source will be readily appreciated in View of the stability therefor alorded both as to rate of ion supply to the accelerator and the output of penetrating radiation. Such mode of pressure stabilization is superior to the crude method heretofore employed of merely allowing gas to leak into an ion source. The cooperation between the controlled ion source and the ionic pump arranged to pump from both ends of the accelerator so as to maintain the latter substantially evacuated will be appreciated, inasmuch as this arrangement increases the likelihood that an ion supplied to` the accelerator will reach the target without collisions on its way. By employing a controlled ion source, and an accelerator that is substantially evacuated at all times, a very high degree of stability of target current is obtained, and this results in a correspondingly high degree of stability in the radiation output of the apparatus.

Whereas FIGURE 2 shows a single pump 36 pumping both from around the target 34 and from the space intermediate the ion-source exit 48 and the accelerator entrance 66, it will be evident that two or more separate pumps may be employed. In such event the pump 36 will pump from around the target 34 and a second pump (not shown) is located in the container 26 substantially axially above the pump 36 and in pumping communication with the annular space 27 by means of openings similar to 72 but arranged to connect the second pump to the annular space 27. In such case the sealing support 23 is of course relocated so as not to obstruct the pumped region of the annular space 27. Alternatively the second pump or a third pump (not shown) may be located in the container 26 substantially axially below the gas reservoir 29 and in pumping communication with the annular space 27 by means of openings similar to 72 but arranged to connect the additional pump to the annular space 27. In such case the sealing support 25 is of course relocated so as not to obstruct the pumped region of the annular space 27.

What I claim as my invention is:

l. In borehole logging apparatus, an elongated housing adapted to be lowered in a borehole, a source of penetrating radiation disposed in said housing, said source of penetrating radiation comprising a linear ion accelerator having electrodes for accelerating ions lengthwise of said housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions to produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, means for maintaining a substantially constant pressure in said ion source, said target and said electrodes and said ion source being disposed in lengthwise alignment in said housing, said target being disposed in said accelerator at the end thereof opposite from said exit of said ion source, and means for maintaining said accelerator substantially evacuated, said lastnamed means comprising an ionic pump connected to said accelerator at a point in lengthwise alignment with and in proximity to said target.

2. In borehole logging apparatus, an elongated housing adapted to be lowered in a borehole, a source of penetrating radiation disposed in said housing, said source of penetrating radiation comprising a linear ion accelerator having a restricted entrance and electrodes for accelerating ions lengthwise of said housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including latomic nuclei reactive with bombarding ions to produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, means for maintaining a substantially constant pressure in said ion source, said target being disposed in said accelerator at the end thereof opposite from said ion source, and means for maintaining said accelerator substantially evacuated, said last named means comprising at least one pump connected to said accelerator at a point proximate said target and having a pumping connection with the region intermediate said exit of said ion source and said entrance to said accelerator.

3. In borehole logging apparatus, an elongated housing adapted to be lowered in a borehole, a source of penetrating radiation disposed in said housing, said source of penetrating radiation comprising a linear ion accelerator having a restricted entrance and electrodes for accelerating ions lengthwise of said housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions to produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, means for maintaining a substantially constant pressure in said ion source, said target being disposed in said accelerator at the end thereof opposite from saidv exit of said ion source, said ion-source exit and said accelerator entrance opening and said accelerator electrodes and said target being disposed substantially in lengthwise alignment in said housing, and means for maintaining said accelerator substantially evacuated, said lastnamed means comprising at least one ionic pump connected to said accelerator at a point proximate said 10 target and having a pumping connection with the region intermediate said exit of said ion source and said entrance to said accelerator.

4. In borehole logging apparatus, an elongated outer housing adapted to be lowered in a borehole, an inner housing Within said outer housing, a'source of penetrating radiation disposed ink said inner housing, said source of penetrating radiation comprising a linear ion accelerator having a restricted entrance and electrodes for accelerating ions lengthwise of said inner housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions to produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, means for maintaining a substantially constant pressure in said ion source, said target being disposed in said accelerator at the end thereof opposite from said ion source, said ion-source exit aud said accelerator entrance and said accelerator electrodes and said target being disposed substantially in lengthwise alignment in said inner housing, means for maintaining said accelerator substantially evacuated, said means comprising at least one pump disposed substantially on the axis of said inner housing proximate said target, a pumping connection to said accelerator proximate said target, and a pumping connection to the region of said inner housing intermediate said exit of said ion source and said entrance to said accelerator.

5. In borehole logging apparatus, an elongated outer housing adapted to be lowered in a borehole, an inner housing within said outer housing, an annular space between said outer housing and said inner housing, a source of penetrating radiation disposed in said inner housing, said source of penetrating radiation comprising a linear ion 'accelerator having a restricted entrance and electrodes for accelerating ions lengthwise of said inner housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions to produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, said target being disposed in said accelerator at the end thereof opposite from the exit of said ion source, said target and said electrodes and said accelerator entrance and said ion-source exit being disposed in lengthwise alignment in said inner housing, an ionic pump connected to said accelerator at a point proximate said target, a irst opening in said inner housing proximate said target and communicating with said annular space, and a second opening in said inner housing intermediate said entrance to said accelerator and said exit of said ion source and said second opening communicating with said annular space.

6. In borehole logging apparatus, an elongated outer housing adapted to be lowered in a borehole, an inner housing within said outer housing, an annular space between said outer housing and said inner housing, a source of penetrating radiation disposed in said inner housing, said source of penetrating radiation comprising a linear ion accelerator having a restricted entrance and electrodes for accelerating ions lengthwise of said inner housing and including a target arranged to be bombarded by ions accelerated by said accelerator, said target including atomic nuclei reactive with bombarding ions t0 produce the penetrating radiation, an ion source having a restricted exit communicating with said accelerator for supplying ions thereto, said target being disposed in said accelerator at the end thereof opposite from the exit of said ion source, said target and said electrodes and said accelerator entrance and said ion-source exit being disposed in lengthwise alignment in said inner housing, an ionic pump in said inner housing connected to said accelerator at a point proximate said target, a first opening in said inner housing proximate said target and communicating with said annular space, and a second opening in said inner housing intermediate said entrance to said accelerator and said exit of said ion source and communicating with said annular space, said annular space and said first and second openings each having cross-sectional areas at least as large as the cross-sectional area of said inner housing.

References Cited in the file of this patent UNITED STATES PATENTS 12 Arnold Nov. 24, 1959 Waer Nov. 15, 1960 Soloway Jan. 3, 1961 Dewan Feb. 28, 1961 Mott Aug. 1, 1961 FOREIGN PATENTS Great Britain Feb. 23, 1955 OTHER REFERENCES Peck et al.: High-Current Cockcraft-Walton Acceler- 

1. IN BOREHOLE LOGGING APPARATUS, AN ELONGATED HOUSING ADAPTED TO BE LOWERED IN A BOREHOLE, A SOURCE OF PENETRATING RADIATION DISPOSED IN SAID HOUSING, SAID SOURCE OF PENETRATING RADIATION COMPRISING A LINEAR ION ACCELERATOR HAVING ELECTRODES FOR ACCELERATING IONS LENGTHWISE OF SAID HOUSING AND INCLUDING A TARGET ARRANGED TO BE BOMBARDED BY IONS ACCELERATED BY SAID ACCELERATOR, SAID TARGET INCLUDING ATOMIC NUCLEI REACTIVE WITH BOMBARDING IONS TO PRODUCE THE PENETRATING RADIATION, AN ION SOURCE HAVING A RESTRICTED EXIT COMMUNICATING WITH SAID ACCELERATOR FOR SUPPLYING IONS THERETO, MEANS FOR MAINTAINING A SUBSTANTIALLY CONSTANT PRESSURE IN SAID ION SOURCE, SAID TARGET AND SAID ELECTRODES AND SAID ION SOURCE BEING DISPOSED 